Monday, November 5, 2012

Understanding Climate Change Part 5 - Ocean Circulation and Climate

Ocean Circulation and Climate

1.Be
able to explain the relationship between subtropical high-pressure cells, the
Coriolis Effect, and the major ocean surface currents.

2.Be
able to explain why the Gulf Stream plays a particularly important role in the
climate of Western Europe.

3.Be
able to explain why water from the Gulf Stream sinks.

4.Be
able to explain how the Gulf Stream drives the global conveyor.

5.Be
able to explain why different masses of water in the deep sea do not readily
mix with each other.

6.Be
able to explain how Ekman transport pulls deep water to the surface.

Introduction

The
climate is the product of complex interactions between solar radiation, the
atmosphere, land, and ocean.You learned
about radiation and the atmosphere in earlier readings and discussions.Now it it’s time to focus on the role of the
ocean in the Earth’s climate.

Ocean Currents

Heat
from solar radiation is absorbed by surface waters of the oceans, and is then transported
around the planet in ocean currents.These currents fall into two main categories: 1) surface currents, and 2) deep-water
currents.We have known about and
monitored surface currents for hundreds of years, but deep-water currents were
discovered only relatively recently, and we are still learning about them and
their affect on climate.

Surface Currents

The
world’s major surface currents are wind-driven.Surface winds circulate around the subtropical high-pressure regions.These high-pressure regions exist as persistent
cells in each major ocean basin, and are the result of air returning to the
surface at the boundary between Hadley and Ferrel Cells.Figure 1 shows the ITCZ and the locations of
the high-pressure cells as they typically appear in July.Also recall that air flows from high-pressure
regions toward low-pressure regions.

A helpful way
to think about high and low pressure regions and the flow of air between them
is to imagine that a high-pressure region develops when dry air piles up on the
surface and forms a mound or hill of air.A low-pressure region, on the other hand, can be imagined as a valley or
depression in the atmosphere next to the surface because air there tends to be
moist and less dense, thus rising away from the surface. So you can imagine air
that is on the top of a high-pressure cell (hill) will flow “downhill” toward a
neighboring low-pressure cell “valley”.

Don’t forget
the Coriolis Effect!This effect causes
moving air to deflect to the right in the northern hemisphere and toward the
left in the southern hemisphere.The
combination of the movement of air between high and low pressure cells and the
Coriolis Effect produces prevailing wind currents indicated by the arrows on
the map in Fig.1.This combination of
air movement and Coriolis Effect gives rise to winds that generally circulate
in a clockwise direction in the northern hemisphere and a counterclockwise
direction in the southern hemisphere.

Figure 1. The ITCZ (red line near the equator), and regional high-pressure
and low- pressure cells, and prevailing surface winds during the month of
July.The size and direction of small
arrows indicate prevailing wind direction and speed. (Image: Modified by Dr. Hipps, USU.)

Just like the
ITCZ, subtropical high-pressure cells move north and south with the seasons.Though they vary in strength with the seasons,
winds around them are quite persistent (Fig. 2).Because surface winds are persistent, they blow
across the ocean surface, create friction, and produce surface ocean currents.

Figure 2.
Seasonal shifts in the location and strength of high-pressure cells.

Clearly the
major currents in each ocean basin surround subtropical high-pressure cells,
one per ocean basin (Fig. 3).Cold-water
currents are indicated in blue and warm-water currents are indicated in
red.Currents shown in black do not move
very far north or south.When a current
moves away from high latitudes it moves cold water toward the equator, and thus
providing a heat sink for equatorial
heat (i.e., a place where heat is removed from the atmosphere).Conversely, when a current moves from lower
latitudes toward higher latitudes, it moves warm water toward the poles and is
a heat source for higher latitudes.In each case heat is being transferred, and
the movement of water impacts regional and global climates.

Figure 3. Prevailing major ocean currents of the world.Currents that carry warm water away from the tropics = red.Currents that carry cold water toward the
tropics = blue.(Image: Arnold J. Bloom, UC Davis.)

There is a
cold-water current running along the west coast of every continent and a
warm-water current running along the east coast of every continent (Fig. 3).This reflects the direction of wind currents
as they flow around subtropical highs.You
should also note that these major surface currents routinely pass under
boundaries between the atmospheric Polar, Ferrel, and Hadley Cells, thus moving
heat away from the tropics to higher latitudes.

The cold current
off of the west coast of North America is most pronounced during the northern
hemisphere summer when the Pacific subtropical high strengthens and moves
closer to North America.Cold water from
this current produces low clouds and cool temperatures during summer months
along most of the west coast of the Pacific Northwest and much of California,
producing cool, moist, temperate rainforest climate.

The Gulf Stream is particularly important
to global climate.The Gulf Stream originates
in the Caribbean Sea, carries warm water northward along the east coast of the
United States, flows across the North Atlantic Ocean, and eventually bathes Western
Europe with that warm water. Because of
this, Europe’s climate is much warmer than its latitude would suggest.Think about this for a minute: Rexburg,
Idaho, is located at about 44oN latitude, and London, England, is
located at about 51oN latitude.This means that London is nearly 400 miles farther north than
Rexburg.To get that far north you would
have to drive to Lethbridge, Alberta, Canada!This means that if everything else were equal London should be colder
than Rexburg, but it’s not.London is
actually cooler in the summer and warmer in the winter than Rexburg (Fig.
4).OK, back to currents and climate.

Figure 4. The average monthly temperatures and monthly average
rainfall in Rexburg, Idaho (left), and London, England (right). (Image:
weather.com.)

Benjamin
Franklin made the first map of the Gulf Stream.He had heard reports of a river of warm water in the Atlantic Ocean, so
during his many trips between North America and Europe he made many temperature
readings at various locations.His map,
based on his temperature measurements, was presented to the British in
1769.His original map is readily
comparable to a modern satellite thermal image of the Gulf Stream (Fig. 5). Franklin’s map is amazingly accurate.This just goes to show that you don’t always have
to have lots of high-tech equipment make good observations and reach powerful
conclusions.

Take notice of the
defined edges of the Gulf Stream.This
current retains its integrity as it travels across the Atlantic Ocean due to
density differences between the warm Gulf Stream water and colder water of the
North Atlantic Ocean.This density
difference restricts mixing between the water masses.The Gulf Stream and most other major currents
therefore retain a high degree of internal consistency, including the retention
of heat, as they move through different latitudes.

If
anything were to slow the rate of the Gulf Stream the climate of Western Europe
would at the very least cool, and at the most affect the global climate by
changing the speed of deep-water currents.

Deep-water Currents

Unlike
surface currents, deep-water currents are not driven by the wind.Instead, the movement of water into and back
up from the deep sea is caused by something called thermohaline circulation.That is, because of differences in seawater density related to
temperature and salinity. Everyone is
familiar with temperature, but salinity may be a new concept for some.Salinity
is the term that refers to the salt content of water.All water, even freshwater, contains some
salt, but seawater contains significantly more.About 3.5% of seawater is made up of salt ions, on average, though the
amount of salt varies slightly from region to region (Fig. 6).

Water density increases
when water temperature drops, salt content increases, or both.However, increasing salt content makes
density grow larger than temperature changes can.This is because water becomes less dense than
liquid water when it freezes.Water is the
only common, naturally occurring compound that does this, and this
characteristic allows life as we know it to exist, but that is topic probably better
left for another time.OK, back to deep-water
currents.

Figure 6. Surface seawater salinity of the world’s oceans.PSU = Practical Salinity Units, and equals
the amount of salt in seawater in parts per thousand.So 34 PSU = 34 parts per thousand salt or
3.4% salt by weight. Note that water in the Caribbean Sea and the Gulf Stream
are more saline than most other surface waters.(Image: Wikimedia Commons.)

Deep-water
circulation of the oceans is caused and driven by density differences of water.The process that drives global-scale deep-water
currents is called thermohaline
circulation.This term refers to the
combination of temperature and salinity that creates the movement of water
because of its density.The deep-water
current that is driven by this process is sometimes called the conveyor belt, the global conveyor, or the Atlantic
conveyor (Fig. 7).

The Atlantic
contribution to the global conveyor is driven primarily by a unique set of
conditions that exist in the North Atlantic Ocean. Seawater in the North Atlantic is particularly
cold, about -2oC or 28oF, and sea ice forms there.About now some of you might be saying, “Hey,
wait a minute, water freezes at 0oC or 32oF!”That’s true, unless that water has a lot of
material dissolved in it, like salt.The
more material water has dissolved in it, the colder it has to be before it will
freeze.This is called freezing point depression. Anyway, when seawater freezes, salt is
excluded.The excluded salt has to go
someplace so it is added to the salinity of water nearby that didn’t freeze. This
water is consequently saltier, and denser than it was before.In the meantime, water that is more saline
than average sweeps northern in the Gulf Stream.This water undergoes evaporation as it flows
north and its salinity also increases.

Figure 7. The Atlantic or global conveyor system.Dense water sinks in the North Atlantic Ocean
as well as in the Antarctic.This water
flows along the ocean floor until it resurfaces in the North Pacific and the
Indian Ocean.Once back at the surface,
water moves across the world’s oceans until it reaches the North Atlantic or
the Arctic again and it sinks.(Image:
Wikimedia Commons.)

Thermohaline
circulation occurs when salty water from the North Atlantic and the Gulf Stream
experience further evaporation and becomes saltier, cools, and becomes dense
enough to sink.Water also sinks in the
Antarctic region where super-cooled water becomes salty as sea ice forms, and
that hypersaline, super-cooled water sinks (Fig. 7).The depth to which a mass of water sinks
depends entirely on the density (temperature and salinity) of water masses
around it.It turns out that the
movement of water in the deep ocean is a much more complex process than we
originally imagined (Fig. 8 and 9), but it wasn’t until recently that
scientists realized the important role of the global conveyor to global
climate.

Figure 8.
Movement of masses of deep water in the Atlantic Ocean (top), Pacific Ocean
(middle), and Indian Ocean (bottom).Note that there are distinct masses of water separated from each other
by density differences determined by temperature and salinity.Denser water masses are closer to the bottom,
and less dense water is closer to the surface.Also note that the mass of water from the Red Sea is fairly deep.Though temperature is not shown, it is a
warmer water mass than masses around it, but it is hypersaline, thus very
dense.(Image: Nybakken and Bertness,
2004.)

Figure 9. A more detailed view of sources and temperature/salinity
characteristics of deep water in the Atlantic Ocean.Note that North Atlantic Deep Water and
bottom water from Antarctica have the same salinity, but Antarctic water is
denser due its lower temperature. (Image: Wikimedia Commons.)

Thermohaline Circulation and Climate

In
recent years we have become more aware of the role of thermohaline circulation
in the global climate.There is compelling
evidence that in past ice ages thermohaline circulation stopped.This would reduce the transport of heat to
higher latitudes.Some concern has
arisen about how the current trend of global warming might affect the Atlantic
conveyor system.We already know that
the largest warming has been and will continue to be observed in the northern
high latitudes.This means the sea ice
and land glaciers will continue to melt, and freshwater will be released into
the North Atlantic.The resulting
reduction in salinity in the North Atlantic could reduce the salinity of
surface waters.This could slow
circulation, since it is the sinking of salty water that drives the process.

In
fact, data already show that salinity in the North Atlantic region has been
decreasing.At the present time, the
best estimates are that circulation will slow somewhat in response to continued
warming, but not stop.This could
possibly cause some regional cooling.At
present it takes water between 1000-1600 years to complete one circuit of the
global conveyor.Of course, more
research needs to be conducted before a definitive prediction can be made.For now, we simply note that the thermohaline
circulation is an important factor in global climate change.

Why
is the global conveyor an important climate factor, aside from moving or not
moving heat?When water from the surface
sinks, water is not the only thing that sinks.Sinking water carries whatever is in it.This includes plankton and dissolved gases.Plankton is made of organic materials – including
carbon.So plankton and any other
organic material that sinks removes carbon from the surface of the earth.It is hypothesized that much of this carbon
is deposited at the bottom of the ocean where it is trapped in ocean
sediments.This is what is referred to
as a carbon sink.Dissolved CO2
is also removed from the water as deep sea organisms, including animals and
small single-celled foraminiferans and radiolarians take it up to secrete
calcium carbonate (CaCO3) shells (Fig. 10).This is also part of the oceanic carbon sink.Plus,
seawater absorbs a huge amount of CO2 directly from the atmosphere.

Another
important form of water movement is referred to as upwelling.Upwelling occurs
when surface currents move along continental margins and Coriolis Effect and winds
push surface waters away from the coast.When this happens, deep water is pulled to the surface to replace
it.The way upwelling was discovered is
pretty interesting.

At
the beginning of the 20th Century, scientists were puzzled about why
icebergs did not move in the direction of the wind or surface currents.Instead icebergs moved at an angle from the
prevailing wind/water direction.One of
the scientists gave the problem to a graduate student whose name was
Ekman.He assembled the proper equations
describing flow that contained Coriolis Effect and friction.He then solved them to show how water flow
changes with depth.The resulting
description is now known as Ekman Transport
or the Ekman Spiral.

A
simple way of visualizing the process is to consider that the flow is being
deflected with depth.Since most of an
iceberg (91% of it, actually) is below the surface, subsurface effects have a
greater effect on it than wind or effects observed right at the water
surface.It turns out that water flow
below the surface does not move not in exactly the same direction as surface
water.In fact, to understand the Ekman
Effect you need to think about water as being in a number of layers stacked on
top of each other.The surface layer is
driven mainly by friction due to wind energy, but the Coriolis Effect causes
the surface layer to deflect slightly to the right in the northern
hemisphere.The water layer just below
the surface also moves by friction with the layer above it, and it too deflects
to the right relative to the layer above it.This friction reduces the amount of energy available to move the layers
farther below, but what we observe is that eventually you will see that once
you reach a certain depth the direction of water movement is 180o
opposite that observed at the surface (Fig. 12).

So, as
water flows along a shoreline, subsurface layers move away from the coast by
the Ekman Spiral, and deeper water is pulled toward the surface (Fig 11).Figure 12 also shows why this is referred to
as the Ekman Spiral.You see the spiral
when you connect the ends of the arrows representing flow.

Figure 11.This figure shows
Ekman Transport of deeper water toward the surface, as it would occur in the
southern hemisphere.How do you know
it’s in the south? (Image: Dr. Hipps,
USU.)

Figure 12. The
Ekman Spiral showing changes in the direction and rate of motion of layers of
water with increasing depth.The
stronger the wind, the deeper the Ekman Spiral will extend.(Image: Wikimedia Commons.)

As a result of upwelling, colder deeper
water rises to the surface.So upwelling
makes surface temperatures colder.These
lower temperatures also affect the climate of coastal regions where upwelling
regularly occurs.Figure 13 shows
regions of coastal upwelling.Upwelling
brings not only colder water to the surface, but also brings nutrients to the
surface.This makes areas with upwelling
biologically productive and economically important as fisheries – not that this
has anything to do with climate, but if climate changed in a way that affected
surface winds and currents, then upwelling would be affected too.

The most
important locations for upwelling are along the west edges of North and South
America, and Africa.Look back at Fig. 3
and you will see that currents in these regions are flowing from high toward
low latitudes, and are cold currents.The upwelling makes surface waters even colder, and has a large effect
on the climate of these regions in moderating warm temperatures.

For
example, the cold California Current flows south along the coast of the Pacific
Northwest and much of California during the summer.This is a result of the subtropical high in
the Pacific intensifying and moving closer to North America.The current experiences upwelling, making
surface waters cold.Low clouds and cool
temperatures characterize the coastal climates during summer in this
region.Without this current, the region
would be much warmer, and this would impact regional climates in terms of
temperature and precipitation.

Source material

Hipps, LE. 2010. Personal
communication and readings produced by Dr. Hipps. Professor of Atmospheric
Science, Department of Plants, Soils, and Climate.Utah State University.